# Fileset [manuscript.pdf](https://mdr.nims.go.jp/filesets/ce9e0280-02b8-43ea-8a6f-437e86a772f5/download) ## Creator [J. Liu](https://orcid.org/0000-0003-2580-7401), [T. Teraji](https://orcid.org/0000-0002-7731-0547), [B. Da](https://orcid.org/0000-0002-0785-8662), Y. Koide ## Rights This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in J. Liu, T. Teraji, B. Da, Y. Koide; Normally-off boron-doped diamond MOSFETs with a breakdown voltage over 1.7 kV. Appl. Phys. Lett. 28 July 2025; 127 (4): 042601 and may be found at https://doi.org/10.1063/5.0278392.[In Copyright](http://rightsstatements.org/vocab/InC/1.0/) ## Other metadata [Normally-off boron-doped diamond MOSFETs with a breakdown voltage over 1.7 kV](https://mdr.nims.go.jp/datasets/e57d0429-2b9e-4e03-97b9-afb293100955) ## Fulltext 1 Normally-off boron-doped diamond MOSFETs with a breakdown voltage over 1.7 1 kV 2 3 J. Liu,1, a) T. Teraji,1 B. Da,2 and Y. Koide1, b) 4 1Research Center for Electronic and Optical Materials, National Institute for Materials 5 Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan 6 2Research and Services Division of Materials Data and Integrated System, NIMS, 1-1 7 Namiki, Tsukuba, Ibaraki 305-0044, Japan 8 9 a) Author to whom correspondence should be addressed; electronic mail: 10 liu.jiangwei@nims.go.jp 11 b) The present affiliation is School of Science and Technology, Mejyo University. 12 13 14 15 16 17 18 19 20 21 22 23 24 https://samurai.nims.go.jp/profiles?unit=wb000https://samurai.nims.go.jp/profiles?unit=kj000https://samurai.nims.go.jp/profiles?unit=kj0002 Abstract 1 Boron-doped diamond (B-diamond) metal-oxide-semiconductor field-effect 2 transistors (MOSFETs) are fabricated on a 150 nm-thick epitaxial layer. The threshold 3 voltage of the B-diamond MOSFET is measured at -8.0 V, indicating a normally-off 4 behavior. Due to the high activation energy for the boron dopants and the relatively thin 5 epitaxial layer, a limited number of holes are formed in the B-diamond and potentially 6 trapped within the Al2O3/B-diamond interface, leading to the normally-off behavior 7 observed in the B-diamond MOSFET. The absolute breakdown voltage for the 8 B-diamond MOSFET is found to exceed 1.7 kV. When divided by the gate-to-drain 9 electrode length of 11.3 μm, the breakdown field is calculated to be 1.52 MV/cm, which 10 is more than two times larger than that of the previous B-diamond MOSFETs. 11 12 13 14 15 16 17 18 19 20 21 22 23 24 3 Diamond, a wide-bandgap semiconductor, exhibits exceptional intrinsic properties in 1 contrast to other semiconductors.1,2 These properties include a high critical breakdown 2 field, large thermal conductivity, high carrier mobility, chemical inertness, and radiation 3 hardness. Diamond-based electronic devices show great promise due to their efficient 4 operation with minimal power loss, high power-frequency capabilities, impressive 5 thermal management, and stable performance in harsh radiation environments. 6 Recently, significant progress has been achieved in the development of p-type 7 hydrogen-terminated diamond (H-diamond)3-8 and boron-doped diamond 8 (B-diamond)9-13 based metal-oxide-semiconductor field-effect transistors (MOSFETs). 9 In the H-diamond channel layer, two-dimensional hole gases form on its surface. While 10 the exact mechanism of this formation is still a topic of debate, two conditions of the 11 presence of surface carbon-hydrogen bonds and negatively charged acceptors are 12 essential.14, 15 These negatively charged acceptors possess the capability to trap electrons 13 from the H-diamond, leading to the creation of hole gases on the H-diamond surface. 14 However, the surface hole gases are not stable at high-temperatures, possibly due to the 15 damage of the negative charge acceptors. The electrical characteristics of H-diamond 16 MOSFETs exhibited a significant degradation as the annealing temperature surpasses 17 300°C.16 18 On the other hand, the B-diamond-based MOSFETs are known to perform well at 19 high-temperatures. In recent research, we employed a relatively flat B-diamond 20 epitaxial layer integrated with a large-area Ohmic contact to create high-performance 21 B-diamond MOSFETs.13, 17 The flat B-diamond surface aids in reducing carrier surface 22 scattering and improving the quality of the gate oxide/B-diamond interface. The 23 large-area Ohmic contact helps in lowering the on-resistance and increasing the output 24 4 current of the B-diamond MOSFETs. 1 The maximum drain current (ID,max) of the B-diamond MOSFETs operating at room 2 temperature and 300°C has been significantly enhanced to -1.2 and -10.9 mA/mm, 3 respectively.17 However, a notable challenge for the B-diamond MOSFETs remains their 4 high threshold voltage (VTH) values, which are recorded at 63.8 V and 31.2 V at room 5 temperature and 300°C, respectively. Previous research has shown that reducing the 6 thickness of the channel layer can substantially decrease VTH for B-diamond 7 MOSFETs.18 In the case of a similar boron doping level, we observed that by reducing 8 the epitaxial layer thickness from 2650 nm to 800 nm, the VTH values for B-diamond 9 MOSFETs could be lowered to below 3.4 V, with the lowest threshold voltage being 0.8 10 V.18 11 In this study, we have reduced the epitaxial layer thickness to approximately 150 nm 12 to investigate the VTH for the B-diamond MOSFET. Our findings reveal that the VTH for 13 the B-diamond MOSFET is measured at -8.0 V, indicating a normally-off behavior. We 14 have also examined the breakdown voltages of the B-diamond MOSFETs at a 15 gate-to-source voltage (VGS) of 0 V. 16 A B-diamond epitaxial layer was grown on an Ib-type (100) diamond substrate using 17 microwave plasma-assisted chemical vapor deposition under the following conditions: 18 microwave power of 1.4 kW, temperature maintained around ~1000 °C, and chamber 19 pressure set at 18.6 kPa.19 The flow rates for the source gases of H2 and CH4 were 49 20 and 1 sccm, respectively. Boron doping was achieved utilizing residual boron present in 21 the chamber from the previous B-diamond growth. The concentration profile of boron 22 atoms in the B-diamond epitaxial layer was analyzed through depth profiling of a 23 secondary ion mass spectrometry (SIMS) technique, as illustrated in Fig. 1(a). The 24 5 thickness of the boron-doped diamond epitaxial layer is approximately 150 nm, with 1 boron concentrations ranging between 1016 to 1017 cm-3. 2 The B-diamond epitaxial layer underwent treatment at 300 °C for 3 hours in a mixed 3 solution of H2SO4 and HNO3 to alter its hydrogen-terminated surface to 4 oxygen-terminated. Subsequently, the B-diamond was coated sequentially with a 5 positive photoresist LOR5A followed by an image reversal photoresist AZ5214E using 6 a spin-coater. The spin speed and time for coating both photoresists were set at 7000 7 rpm and 1 second, respectively. Baking parameters for LOR5A were a temperature of 8 180 °C and a duration of 5 minutes, while for AZ5214E, the temperature and time were 9 110 °C and 2 minutes, respectively. The exposure and development processes were 10 carried out using a DL-1000 scanning maskless lithography system and 11 tetramethylammonium hydroxide solution with a concentration of 2.38%, respectively. 12 The developing process in the TMAH solution took approximately 2.0 minutes. 13 The source/drain electrodes, composed of a Ti/Au bilayer (10/150 nm), were 14 deposited on the B-diamond using an electron-gun evaporation system. The chamber 15 pressure during the evaporation of the Ti/Au bilayer was maintained at approximately 16 10-6 Pa, with evaporation rates set at 1 Å/s for Ti and 2 Å/s for Au. Subsequently, the 17 electrodes underwent annealing at 550 °C for 20 minutes in an Ar atmosphere using a 18 rapid thermal annealing system to establish Ohmic contacts. An Al2O3 gate oxide layer 19 was then deposited via atomic layer deposition technique at 250 °C utilizing Al(CH3)3 20 and ozone precursors. For the gate electrode, a Ti/Au bilayer (10/150 nm) was 21 employed. A cross-sectional high-resolution transmission electron microscope (TEM) 22 image of the Al2O3/B-diamond interface is presented in Fig. 1 (b), revealing an abrupt 23 interface. The thickness of the Al2O3 film was measured to be 21.2 nm. 24 6 To access the source/drain electrodes, windows were opened by etching the Al2O3 1 film using a capacitively coupled plasma reactive-ion etching system in a CHF3 + Ar 2 atmosphere. The etching process involved a plasma power of 100 W, chamber pressure 3 of 3.0 Pa, CHF3 flow rate of 10 sccm, and Ar flow rate of 40 sccm. The electrical 4 properties of the B-diamond MOSFETs were characterized using a Grail 10-5-LV-HTV 5 prober system. 6 Figures 2(a) and 2(b) shows a scanning electron microscopy image and schematic 7 diagram of the B-diamond MOSFET, respectively. The diameter for the drain electrode 8 is 398.2 μm. The gate width can be calculated as 1250.3 μm. The gate length is 3.0 μm. 9 The interspatial lengths for the gate-to-source and gate-to-drain electrodes (LG-S and 10 LG-D) are 5.0 and 11.3 μm, respectively. 11 Figure 3(a) depicts the ID as a function of drain voltage (VD) for the B-diamond 12 MOSFET. The VGS ranges from -16.0 to 40.0 V in increments of +1.0 V, showcasing 13 distinct saturation regions and p-type characteristics. The absolute ID,max for the 14 B-diamond MOSFET is 0.12 μA/mm, significantly lower than the maximum value of 15 1.2 mA/mm reported previously.11, 17 This difference could be attributed to the much 16 thinner epitaxial layer thickness (from 2650 nm to 150 nm), the increased LG, and the 17 larger LG-D. The on/off ratio for the B-diamond MOSFET was determined to be greater 18 than 104. The off-current is below 10-6 μA/mm, consistent with the previous report.17 19 However, due to the low on-current observed in the current MOSFET, its on/off ratio is 20 notably lower than the previous report of 109. 17 21 Figure 4(a) illustrates the - –VGS characteristic for the B-diamond MOSFET. The 22 VTH is determined to be -8.0 V, indicating a normally-off behavior. This marks the first 23 demonstration of a normally-off p-type B-diamond MOSFET. There are several reports 24 7 for the normally-off p-type H-diamond or silicon-terminated diamond (Si-diamond) 1 based MOSFETs.20-26 The formation of normally-off characteristics for the H-diamond 2 and Si-diamond MOSFETs can be achieved by modifying the two necessary conditions 3 of carbon-hydrogen bonds and negatively charged acceptors for the channel layers. As 4 the carbon-hydrogen bonds damaged by surface etching changing to carbon-oxygen 5 bonds partially24-26 and the negatively charges acceptorts compensated by positive 6 charges in the oxide insulators,21-23 the hole gases in the H-diamond and Si-diamond 7 channel layers cannot formed. These are the formation mechanism for the normally-off 8 behaviors of the H-diamond and Si-diamond MOSFETs. On the other hand, a selective 9 growth of a p-type epitaxial layer on the n-type diamond body with 10 hydroxy-termination can also lead to the formation of normally-off characteristics for 11 the diamond MOSFETs.27, 28 12 On the other hand, previous reports on the p-type B-diamond MOSFETs all exhibited 13 normally-on behaviors.9-13, 17 In this study, we have successfully developed a 14 normally-off B-diamond MOSFET by reducing the epitaxial thickness. The inset figure 15 in Fig. 4(a) illustrates the schematic diagram depicting the formation mechanism of the 16 normally-off characteristic of the B-diamond MOSFET. Due to the high activation 17 energy required for boron dopants and the thin epitaxial layer thickness, only a limited 18 number of holes are generated in the B-diamond. Furthermore, the presence of defects 19 and oxygen vacancies at the Al2O3/B-diamond interface facilitates the potential capture 20 of these limited holes within the interface, providing an explanation for the normally-off 21 behavior of the B-diamond MOSFET. 22 The breakdown voltage (VB) values for two B-diamond MOSFETs were measured 23 and are depicted in Fig. 4(b) as -1612 and -1718 V, respectively. Dividing these values 24 8 by the LG-D (11.3 μm) yields breakdown fields of 1.43 and 1.52 MV/cm, respectively. 1 These values are more than two times larger than the 0.67 MV/cm of the previous 2 reported B-diamond MOSFET with VB/LG-D of 200 V/3.0 μm.29 Furthermore, they are 3 better than the breakdown fields of other devices, including the B-diamond 4 metal-semicondutor FET of 1.39 and 0.51 MV/cm with the VB/LG-D values of 693 V/5.0 5 μm and 1530 V/30 μm, 30 and the H-diamond MOSFET of 0.73 MV/cm with the 6 VB/LG-D of 3659 V/50 μm.31 7 In this study, the B-diamond MOSFETs were fabricated on a 150 nm-thick thin 8 epitaxial layer. The VTH of the B-diamond MOSFET was recorded at -8.0 V, indicating 9 the normally-off behavior. The limited holes in the thin B-diamond epitaxial layer were 10 formed and captured within the Al2O3/B-diamond interface potentially, explaining the 11 normally-off behavior. The absolute VB and electrical field for the B-diamond MOSFET 12 were measured to be 1718 V and 1.52 MV/cm, respectively. 13 14 This work is supported by the JSPS KAKENHI Projects (JP23K03966, 20H05661, 15 and JP20H00313), MEXT Q-LEAP (JPMXS0118068379), JST CREST (JPMJCR1773), 16 JST Moonshot R&D (JPMJMS2062), MIC R&D for construction of a global quantum 17 cryptography network (JPMI00316), and ARIM (23WS0311 and 23NM5006) of the 18 Ministry of Education, Culture, Sports, Science and Technology, Japan. 19 20 Data Availability Statements 21 The data that support the findings of this study are available from the corresponding 22 author upon reasonable request. 23 24 9 References 1 1. C. J. H. Wort and R. S. Balmer, Mater. Today 11, 22 (2008). 2 2. H. Umezawa, Mater. Sci. Semi. Proc. 78, 147 (2018). 3 3. K. Hirama, H. Sato, Y. Harada, H. Yamamoto, and M. Kasu, Jpn. J. Appl. Phys. 51, 4 090112 (2012). 5 4. J. Liu, H. Ohsato, M. Y. Liao, M. Imura, E. Watanabe, and Y. 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Inset figure shows 13 schematic diagram for the formation mechanism of normally-off characteristic of the 14 B-diamond MOSFET. (b) Breakdown voltage measurement for the B-diamond 15 MOSFETs. 16 17 18 19 20 21 22 23 13 0 50 100 150 200 25010151016101710181019 Depth (nm)Concentration (Atom cm‒3)(a) (b)DiamondAl2O35 nmTi 1 2 Liu et al., Figure 1 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 14 Ib-type (100) diamond substrateSourceGateAl2O3 (20.2 nm) DrainBoron-doped diamond3.0 μm5.0 μm 11.3 μm 200 μmGate DrainSource(a) (b) 1 2 3 Liu et al., Figure 2 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 15 0 -2 -4 -6 -8 -10 -12 -14 -160.00-0.02-0.04-0.06-0.08-0.10-0.12-0.14 I D (μA/mm)VD (V)VGS: –16.0 ~ 40 VSteps: +1.0 V(a)4 0 -4 -8 -12 -1610-710-610-510-410-310-2 VGS (V)I D (μA/mm)On/off: ~104(b)VGS＝ –16.0 V 1 2 Liu et al., Figure 3 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 16 0 -2 -4 -6 -8 -10 -12 -14 -160.00-0.02-0.04-0.06-0.08 VTH＝–8.0 VVGS (V)(μA0.5/mm0.5)(a)Al2O3B-diamond+ ++ ++h+ h+ h+ h+ h+ h+ h+ h+(b)0 -400 -800 -1200 -1600 -200010-410-310-210-1100101102 I D (μA/mm)VD (V)–1612 V–1718 V 1 2 Liu et al., Figure 4 3 4 5 6 7 8 9 10 11 12 13